The present invention relates to devices for providing a fluid-tight seal or block in a tubular member such as a conduit or nozzle or the like. The invention has particular application to providing a dam or seal in the primary fluid nozzles of a nuclear steam generator.
A typical nuclear steam generator comprises a vertically oriented shell or vessel. A tube sheet toward the lower end of the vessel supports a tube bundle comprising a plurality of tubes, each generally in the shape of an inverted U. The portion of the vessel beneath the tube sheet is divided by a partition into inlet and outlet plenums which are respectively disposed immediately beneath the vertical legs of the tube bundle. The vessel is provided with primary fluid inlet and outlet nozzles respectively communicating with the plenums, each nozzle projecting outwardly of the vessel and commonly having a frustoconical portion and a circularly cylindrical portion. The nozzles are connected by conduits to an associated nuclear reactor vessel.
Primary fluid, having been heated by circulation through the reactor, enters the steam generator vessel through the primary inlet nozzle to the inlet plenum and from there flows upwardly through the tube bundle to the outlet plenum and then back through the outlet nozzle to the reactor core. The tube bundle above the tube sheet is immersed in a secondary fluid, such as water, the tube bundle serving as a heat exchanger for converting the secondary water to steam, which is then used for generating electricity in the usual manner.
Periodically, it is necessary to shut down the reactor for refueling. This is usually a convenient time for servicing the nuclear steam generator. In this regard, reactor is drained to below the level of the inlet and outlet nozzles of the steam generator vessel. The tube bundle and inlet and outlet plena are also drained and dams are then installed in the inlet and outlet nozzles to block them. Once these dams are in place, the reactor can then be refilled with water for the refueling operation, without interfering with service activities being conducted in the steam generator vessel. The water serves as both a shield and a moderator during the refueling operation, but the dams in the steam generator nozzles prevent flow of this material into the steam generator vessel. Also, the steam generating vessel can be treated in a chemical decontamination procedure without contaminating the primary fluid conduits and the nuclear reactor.
Heretofore, there have been generally two types of fixed or static nozzle dams utilized. One type involves drilling of holes in the inner surface of the nozzle to receive radially extendable pins from a sectional seal or dam plate which is inserted in the nozzle. The plate carries a seal diaphragm which may have radially inflatable portions around the periphery thereof to frictionally engage the inner surface of the nozzle.
Another type of dam involves the mounting of an attachment ring inside the nozzle at its distal end, as by welding, this ring serving as an attachment ring on which a nozzle dam plate can be attached, by suitable fasteners such as bolts. Both of these techniques involve modification of the nozzle, and if the modification is effected after the steam generator is in service, man-rem exposure is involved. Also, both of these types of dams can affect the flow of primary fluid through the nozzle, particularly in the case of the attachment ring which projects radially into the nozzle and could alter the flow path.
It is also known to utilize expandable bags or diaphragms or the like which are inserted in the nozzle and expanded to plug the nozzle. But such expandable seals can sustain only a very limited pressure differential on the opposite sides thereof, since they do not adhere particularly well to the nozzle inner surface and are not anchored thereto. Thus, they are useful in only very limited applications.